WORKSHEET ON DISCHARGE TUBES
AND ELECTRIC & MAGNETIC FIELDS

Answers
are supplied. Simply click on the word "answers" at the end of
each question.

1Explain why the cathode ray beam in the diagram below broadens along its
path. (Answers)

2Deduce the sign of the charge on the cathode rays from the deflection of
the rays shown in the following diagram. (Answers)

3If a very high voltage is applied across the ends of a Geissler tube and
the air pressure inside the tube is gradually decreased, a pinkish/purplish glow
appears in the tube and then this glow breaks up into a series of bright and
dark bands called striations.See the diagram below.

5An electron travelling at 3 m/s passes between the poles of a magnet as
shown below.

If the magnetic flux density is 0.5 T, determine the magnitude and direction
of the force on the electron.Charge
on electron = - 1.6 x 10-19 C. (Answers)

6Define the term “crossed fields” as it is usually applied in
Physics. (Answers)

7A charged particle moves in a straight line through a region of space.Does this necessarily imply that there is no magnetic field
present? (Answers)

8A boy stands in front of the screen of an old black and white television
set while there is a picture on the screen.He places a bar magnet on top of the TV set so that the magnetic field is
directed down the front of the screen.Describe
what you would expect to happen to the picture on the screen.Explain this effect. (Answers)

9The diagram below shows an oscilloscope trace.

(a)To which deflection plates (the x or y) is the time-base voltage applied
in an oscilloscope?

A uniform electric field of strength 200 N/C exists between two charged
parallel plates each of which is 0.1 m long.The separation between the plates is 0.1 m.An electron with a speed of 3 x 106 m/s enters the
field perpendicular to the field lines.

(a)Calculate the acceleration of the electron whilst between the plates.Ignore the effects of gravity.Mass
of electron = 9.11 x 10-31 kg.

(b)Determine the time the electron spends between the plates.

(c)Calculate the distance the electron moves vertically between the plates.

1.Due to the Coulomb repulsive forces experienced by each electron in the
beam due to the presence of other electrons.

2.Charge is negative by Fleming’s Left Hand Rule (or equivalent hand
rule).For Fleming’s LHR, thumb
of left hand points down in the observed direction of the force on the charges,
index finger points into page in direction of magnetic field and therefore the
second finger points to the left showing the direction of conventional current
(positive charge) flow.Therefore,
since the cathode ray beam moves to the right, it seems that it consists of
negative charges.

3.(a) The glow in the tube is partly produced by light emitted from gas
molecules and atoms when electrons within them de-excite by dropping back from
high energy levels to lower levels. The glow is also partly due to the
recombination of electrons and positive ions that occurs during collisions of
these particles.The electrons are
captured by the ions and de-excite by emitting light energy.

(b) Striations are caused by alternate ionizations and recombinations in the
tube.The dark bands are positions
where ionizations are occurring mainly due to collisions between ions and
neutral atoms and molecules.The
gas atoms and molecules absorb energy, which results in the excitation of
electrons within them and in many cases the ionization of the atoms and
molecules.Thus, no light is
emitted.The bright bands are
places where light is being emitted, either by the de-excitation of electrons
during recombination with positive ions or by the de-excitation of electrons
within excited atoms and molecules.(Note
that the physics of gas discharge tubes is quite complex and that much more
could be said here.However, it is
probably true to say that for our present Syllabus the first sentence of this
answer is sufficient to answer question 3 (b).

4.Any three of the following:

a.Are always the same regardless of which metal is used as the cathode.

b.Travel in straight lines perpendicular to the surface emitting them.

c.Are deflected by magnetic fields as if they were negatively charged
particles.

d.Are deflected by electric fields.

e.Cause glass to fluoresce.

f.Carry energy and momentum.

g.Produce some chemical reactions similar to the reactions produced by
light – some silver salts change colour when struck by cathode rays.

6.The term “crossed fields” refers to electric and magnetic fields at
right angles to each other.

7.No.There may be a magnetic
field present and the charge is travelling parallel to or anti-parallel to the
direction of the field.Such a
charge would experience no force due to the field.The other alternative is that the charge could be travelling in an area
of crossed electric and magnetic fields, where the net force on the charge is
zero.

8.The picture would be distorted because the electrons travelling to the
screen would experience an extra force as they passed through the magnetic field
of the bar magnet.This force would
be to the left of the screen as seen by the boy standing in front of the screen
and facing the screen.(WARNING: DO
NOT experiment with this on your parents’ TV.External magnetic fields can change the settings of electronic components
within the TV – eg colour settings – and also on occasions damage such
components.)

9.(a) The time-base is applied to the X-deflection plates (the plates that
are arranged vertically within the oscilloscope).

(b) From the trace, the period = 3.9 ms.Therefore,
since the frequency is the reciprocal of the period, frequency = 256.4 Hz.

(c) 3.5 V or 3.6 V – either answer will do.

10.(a) From E = V/d, E = 50 000 V/m.(b) From F = qE, F = 100 000 N.

11.(a) Use F = qE to calculate the force on the electron
and then use a = F/m
to determine the acceleration.a
= 3.51 x 1013 ms-2, up towards positive plate.Note also, some people worry as soon as they get an acceleration greater
than 3 x 108 ms-2.Please
don’t!Velocity and acceleration are different quantities.Although, according to Special Relativity Theory it is not possible for
any physical object in the universe to exceed the speed of light (3 x 108ms-1),
it is absolutely fine to have an acceleration whose magnitude is greater than 3
x 108 ms-2.You
simply need to appreciate that no matter how long you apply the force that is
producing such an acceleration, you can never exceed the velocity of light.

(b) Realise that there is no acceleration acting horizontally.The force on the electron due to the electric field is vertical in
direction.So, the horizontal
distance through the plates is 0.1
m and the speed of the electron is 3 x 106 ms-1.Use time = distance/speed and therefore, time = 3.33
x 10-8 s.

(c) Apply s = ut +
0.5at2 vertically.The
electron has no initial vertical velocity and is between the plates for 3.33
x 10-8 s.The
acceleration is as calculated in part (a) and so the distance moved vertically
by the electron is 0.019
m or 1.9 cm.

12.Use the same method as in 11 (a) to determine the acceleration of the
positron.Then apply v
= u + at to obtain v = 8.8 x 105 ms-1 in the direction of the
electric field.

13.Use F = q v B sin q to obtain F = 1.13 x 10-13 N, up out
of page.

14.(a) Up the page (by Fleming’s LHR).

(b) The force on the electron due to the magnetic field is causing the electron
to execute circular motion in the field.So
this force can also be described by the centripetal force formula.Use mv2/r
= qvB to obtain r
= 2.63 x 10-5 m or 26.3 mm.

(c) The electron is executing circular motion.It is therefore accelerating.Maxwell’s
Theory of Electromagnetism tells us that an accelerating electric charge
produces a changing electric field, which in turn produces a changing magnetic
field, which in turn produces another changing electric field and so on.In short, the electron loses energy by emitting it as electromagnetic
radiation.

15.(a) P must be attached to the negative terminal of the high voltage
supply.P must be made the cathode
so that electrons produced by ionisation of gas molecules due to the strong
electric field between the cathode and anode (Q) can be accelerated down the
tube in the direction from P to Q.

(b) The glass tube must be sealed so that it can be evacuated to low gas
pressure thereby ensuring a relatively free path for the cathode rays
(electrons) to travel all the way down to the other end of the tube.

(c) These coils produce a magnetic field transverse to the path of the cathode
rays thereby allowing the experimenter to apply vertical magnetic forces to the
cathode ray beam.This was an
essential feature of Thomson’s experiment.He applied the magnetic field to the cathode rays, which caused them to
move in a circular path as they passed through the field.By writing the force acting on the cathode rays as both a
magnetic force and a centripetal force, he was able to derive a mathematical
equation for the charge to mass ratio of the electron.He also noted the position of the beam on the end of the tube.From this the radius of curvature of the cathode rays in the field could
be determined.Thomson then
balanced this downwards magnetic force on the cathode rays with an upwards force
produced by the electric field between the charged plates.This enabled him to determine the velocity of the cathode rays through
the tube and then to calculate the charge to mass ratio of the electron from the
equation for this that he had derived.

(d) The tape measure at J allowed Thomson to measure how far the cathode ray
beam was displaced on the end of the tube when the magnetic field was turned on.This was then used to calculate the radius of curvature of the cathode
rays in the magnetic field.

(e) The cathode ray oscilloscope and the television.(There are plenty of others.)

WORKSHEET ON THE DAWN OF THE
QUANTUM ERA

1.Perform an
investigation to demonstrate the production and reception of radio waves.(In class)

2.Identify data
sources, gather, process and analyse information and use available evidence to
assess Einstein’s contribution to quantum theory and its relation to black
body radiation.(Homework) Hint
- see below

3.Identify data
sources, gather, process and present information to summarize the use of the
photoelectric effect inphotocells.

Einstein's Contribution to
Quantum Theory & Its Relation To Blackbody Radiation - a few points

·Revolutionary, extremely powerful, insightful, dynamic, foundational, inspiring,
valuable, essential contribution – don’t use all of these adjectives – but
make sure you make an assessment of the contribution. Einstein's
work, more than that of any other Physicist, prompted the development of the
Quantum Theory. (Ref: Subtle is the Lord, The Science and The Life of
Albert Einstein by Abraham Pais)

·Planck introduced E = hf – the energy of a quantum is
proportional to its frequency.Planck,
however, thought of this relationship as a mathematical device not a physical
reality.(Ref: Quantum Physics by
Eisberg & Resnick)

·Einstein used E = hf and his ideas on atomic behaviour to develop
a theory to describe the properties of light on the inside of a black body
radiator (an oven).Light is
produced when the atoms in the walls of the oven heat up and vibrate.These studies led Einstein to propose the Particle Theory of Light.
This theory has two postulates: (i) that light consists of particles (photons) each of energy E = hf;
and (ii) that when light is absorbed or emitted by matter, it is absorbed or
emitted in discrete quantities. This theory was revolutionary. It
upset all existing ideas about the interaction between light and matter.
Although not accepted widely at first, this theory led eventually to the
development of the Quantum Theory. (Ref: Subtle is
the Lord, The Science and The Life of Albert Einstein by Abraham Pais)
[The paper in which Einstein proposed the idea of energy quanta is titled
"On a Heuristic Viewpoint Concerning the Production and Transformation of
Light", March 1905.]

·Einstein used his ideas on the quantization of light to successfully explain the
photoelectric effect that could not be completely explained using classical
physics.

·Thus, Einstein effectively showed that the EM radiation from a
black body radiator is quantized and that this fact then explains other physical
phenomena such as photoelectric effect.(Ref: Three Roads to Quantum Gravity by L Smolin)

·Einstein’s work gradually convinced other physicists that
quantization was a real effect not just a mathematical device.Thus, Einstein is probably more responsible for the birth of quantum
theory than Planck.

WORKSHEET ON
SOLID STATE PHYSICS

Perform an investigation to model the
behaviour of semiconductors, including the creation of a hole or positive charge
on the atom that has lost the electron and the movement of electrons and holes
in opposite directions when an electric field is applied across the
semiconductor. (In class)

Gather,
process and present secondary information to discuss how shortcomings in
available communication technology lead to an increased knowledge of the properties of
materials with particular reference to the invention of the transistor.
(Homework – check out my Website – Useful Links page – under
“From Ideas To Implementation” heading – try the “Semiconductors” link
that takes you to the “Transistorized Website” & the “Shockley,
Brattain & Bardeen” link.) View the sample
answer to this question.

Identify data sources, gather, process, analyse information and use
available evidence to assess the impact on society of the invention of
transistors, with particular reference to their use in microchips (integrated
circuits) and microprocessors. (Homework)

Identify data
sources, gather, process and present information to summarize the effect of
light on semiconductors in solar cells. (Homework)

3.Analyse
information to explain why a magnet is able to hover above a superconducting
material that has reached the temperature at which it is superconducting.(Homework)

4.Gather and
process information to describe how superconductors and the effects of magnetic
fields have been applied to develop a maglev train.(Homework)

5.Process
information to discuss possible applications of superconductivity and the
effects of those applications on computers, generators and motors and
transmission of electricity through power
grids.(Homework)

EXAMPLE
ANSWER

Gather, process and present
secondary information to discuss how shortcomings in available communication technology lead
to an increased knowledge of the properties of materials with particular
reference to the invention of the transistor.

The invention of the transistor is
a good example of what often happens in science, where shortcomings in available
technology stimulate further research that eventually leads to improved
technology. In the 1930’s and
1940’s, a prime motive of the scientists at Bell Laboratories in the USA was
to replace the old, unreliable mechanical relays (electromagnetic switches with
moving parts) in telephone exchanges with electronic relays.
Vacuum tubes were used for this purpose but they took up a huge amount of
space and large exchanges with many vacuum tubes required constant maintenance.
Something smaller was required.

Semiconductor crystals had been
used as current-rectifiers in radios in the 1930’s but the physics of how they
worked was not understood. Shortcomings
in this communication technology, namely the ease with which these rectifiers could burn out,
led directly to increased knowledge of the properties of materials.
Work by Russell Ohl in 1939 and 1940 led to his discovery of the effect
of a barrier in a silicon crystal with different levels of purity on either side
of the barrier – the silicon p-n junction.
Work by Karl Lark-Horovitz and Seymour Benzer led to the discovery of the
excellent rectifying properties of germanium crystal and that adding impurities
to the germanium could greatly enhance its current-carrying capacity.

Using this improved knowledge of
materials, Walter Brattain and John Bardeen used germanium to develop the first
transistor in 1948. Called the point-contact
transistor, this was the first semiconductor amplifier.
Although this was a huge breakthrough, these transistors had their own shortcomings,
which in turn led to increased knowledge of the properties of materials.

William Shockley, also in 1948,
proposed a different design for the transistor. He suggested a sandwich structure in which two n-type
semiconductor layers were separated by a p-type layer.
The successful operation of the point-contact transistor could be
explained by assuming that the current flowed around the surface of the
germanium. For Shockley’s design
to work, current would have to flow through the crystal.
Further research was necessary.

Richard Haynes showed that current
could indeed flow through a crystal of germanium. His research also showed that the layer in the middle had to
be very thin and very pure. After a
great deal of effort, Gordon Teal developed a technique for producing very pure
single crystals of germanium and showed that single crystals were better
current-carriers than slivers cut from a larger ingot of many crystals.

Next Teal and Morgan Sparks teamed
up to develop a technique for producing a single crystal that had n-type
material at each end and p-type material in the middle.
By 1950, they had achieved this goal and the first junction transistor
was produced.

At first these transistors could
only amplify very small signals. The
problem was the thickness of the middle p-type layer.
This led Sparks to improve the single crystal manufacture technique and
by 1951 he had produced single crystals whose middle p-type layers were thinner
than a sheet of paper. This greatly
improved the performance of the transistor.

Transistors made from germanium
still had one large shortcoming. As
the germanium heated up the transistor produced too many free electrons, which
effectively stopped the transistor working.
In 1954, Gordon Teal again came to the rescue finally producing a working
silicon transistor, after several years of experimentation.

Clearly, the history of the
invention of the transistor shows how shortcomings in the available
communication technology led
to an increased knowledge of the properties of materials.

(This response could be summarised
to produce a much shorter but still appropriate answer to this question.
More detail than required has been given in this answer.)